Monday, June 9, 2014

Increased Ice Extent This Autumn?

An intriguing long-range forecast has emerged recently from the NWS Climate Forecast System (CFSv2) model, which makes predictions of global climate conditions up to 9 months into the future. The model is quite sophisticated and often does provide considerable insight into likely future climate anomalies; but recently the model has increasingly portrayed a scenario that seems implausible at first glance; the model is showing a notable uptick in Arctic sea ice extent (compared to recent years) during this year's melt season. Given the remarkable and persistent warmth in the Arctic in recent years, and the strong trend for increasing autumn melt-out, it would be quite an interesting change if this year brought significantly higher ice extent - and it would be a considerable success for the CFSv2 model.

The chart below shows the most recent Arctic sea ice extent forecast from the model, showing a predicted mean extent of over 6.5 x106 km2 in September. The observed September extent since 1979 is shown in the second figure below; the big melt years were 2007 and 2012. Last year saw a large rebound, but if the CFSv2 forecast is correct, the ice area this year will jump back up to a level not seen since 2001; this would certainly generate a great deal of discussion and interest in the climate community and beyond.

The maps below show the spatial distribution of the CFSv2 anomalies in the next three months. Curiously, the model is showing anomalous ice cover persisting in the coastal margins of the Arctic Ocean from the Laptev Sea all the way around to the Beaufort Sea and the Canadian Arctic Archipelago; but the model shows a lack of sea ice farther north. It is not clear if this is at all realistic; but I would note that last year the model performed rather well in predicting the September ice extent, and so I don't think the latest forecast can be dismissed out of hand. The last chart below show the forecast from this time last year; the September 2013 ice extent verified at 5.4 x106 km2, and so the forecast from June was just about spot on.

For those who may be interesting in following the CFSv2 forecasts, here is the website (scroll to the bottom for sea ice):

I probably speak for most people in that it would be a great thing if the forecast verifies - even if it only delays the inevitable. The more drawn out that the sea ice melt-out is, the more opportunity for plants and animals to adapt to a new ecological setting.

I am curious to what extent their forecast is based on ENSO and PDO. It seems like most 3-18 month forecasts are heavily weighted toward teleconnection correlations.

Here is a blockquote from a recent article (embargoed) in the Journal of Climate ( http://cfs.ncep.noaa.gov/cfsv2.info/CFSv2_paper.pdf ):

"The model shows a consistent high bias in its forecasts of September ice extent. The corresponding predicted model variability at the three different lead times is shown in Fig. 11. The variability from the model prediction is underestimated near the mean September ice pack and overestimated outside the observed mean September ice pack. Although the CFSv2 captured the observed seasonal cycle, long-term trend, and interannual variability to some extent, large errors exist in its representation of the observed mean state and anomalies, as shown in Figs. 10 and 11. Therefore in the CFSv2, when the sea ice predictions are used for practical applications, bias correction is necessary. The bias can be obtained from the hindcast data for the period 1982- 2010, which are available from NCDC."

It goes on to say that when the known bias is used to modify the CFSv2 forecast, it actually outperforms most other models.

Brian, thanks for digging this up - this is excellent information. I'm certainly going to have to obtain the hindcast data and check out the size of the bias.

The CFSv2 is a global coupled ocean-atmosphere model, so it does not forecast ENSO and PDO independently but rather it models the SST variations that contribute to these large-scale indices. Most of these global circulation models have some ability to predict ENSO out to 12 months or so, but it is an interesting question as to how faithfully they represent the interactions between the Arctic and the mid-latitude oceans/atmosphere.

I forgot to note potential changes in ocean salinity and temperature in the above comment, mainly due to the dilution of existing saline with fresh water from ice melt or terrestrial input.

For an example refer to the hypothesized effects of ancient Lake Agassiz (central Canada) periodically dumping fresh water into the saline Arctic ocean on Arctic and North Atlantic Ocean climate, and it's hypothesized contribution to the Younger Dryas cooling event.